1. Field of the Invention
This invention relates generally to regulation of charge pumps, and more specifically to a skip-mode regulator for a charge pump.
2. Related Art
A charge pump delivers an output voltage VOUT at a desired value to a load that includes a load, or output, capacitance COUT. A charge pump regulator attempts to maintain VOUT at the desired value by selectively enabling and disabling the charge pump. The value of VOUT from a charge pump varies and has a maximum value and a minimum value. The maximum value occurs when VOUT overshoots a reference voltage VREF of the charge pump. The minimum value occurs when VOUT undershoots the reference voltage VREF of the charge pump. The response time of the regulator after overshoot (td_off) is defined as a maximum delay time for the regulator to disable the charge pump after VOUT becomes higher than VREF. The response time of the regulator after undershoot (td_on) is defined as a maximum delay time for the regulator to enable the charge pump after VOUT becomes smaller than VREF. The response time of the regulator (td_reg) is defined as the maximum delay time before the regulator disables the charge pump after VOUT becomes higher than VREF plus the maximum delay time before the regulator enables the charge pump after VOUT becomes smaller than VREF. In other words, td_reg=td_off+td_on. Typically, td_on=td_off.
A difference between the maximum value and the minimum value of VOUT is defined as an output voltage ripple VRIPPLE. In a charge pump regulator that uses latched comparators, output voltage ripple is directly proportional to a response time td_reg of the regulator and a current through the load ILOAD, and inversely proportional to COUT. In other words, VRIPPLE∝ILOAD·td_reg/COUT. As can be seen from the preceding equation, the output capacitance COUT and the response time of the regulator td_reg affect the magnitude of the output voltage ripple; therefore, some known charge pumps reduce the magnitude of the output voltage ripple by increasing the output capacitance and/or decreasing the response time.
Because of the discrete sampling nature of a latched comparator, there is a possibility that the comparator does not latch a resolved signal during a first latching interval. Therefore, the result of a comparison is pushed out to a subsequent latching interval, which disadvantageously increases the magnitude of the overshoot or the undershoot of the output of the charge pump.